Those findings come from lab tests on mice, but the researchers say that erectile dysfunction (ED) is common among men with obstructive sleep apnea, a condition in which breathing stops several times per night.

In the new study, male mice showed less sexual activity within a week of being exposed to brief but chronic bouts of oxygen deprivation during sleep. That includes attempts to mate with female mice and spontaneous erections.

The researchers, who included the University of Louisville's Galia Soukhova-O'Hare, ground up Cialis pills and mixed them into peanut butter for the male mice. The mice became more sexually active, but not quite as active as they had been before the study started.

How were sleep apnea and ED linked? Testosterone wasn't the problem; testosterone levels were unaffected by sleep apnea. And there weren't any problems with the mice's anatomy, either.

But the mice did have lower levels of an enzyme needed to make nitric oxide during the experiment, and they may have needed nitric oxide to help with blood flow for erections.

The researchers stop short of recommending ED drugs for men with sleep apnea, and they also note that using a continuous positive airway pressure (CPAP) machine to treat sleep apnea can help with erectile dysfunction.

The findings appear in the Sept. 15 edition of the American Journal of Respiratory and Critical Care Medicine.